The Image Portfolio holds only pixel-based layers. If you want to add a shape, Watercolor, Liquid Ink, or dynamic layer to the Image Portfolio, you must first convert it to a default layer. In the Layers panel, click the Layer options button panel options, and choose Convert To Default Layer.

To work-around this, select all, then copy, and then paste. The contents of the subsequent PNGs can be then added to the portfolio.